Canadian Journal Journal canadien of Chemistry de chimie Published by Publie par THE NATIONAL RESEARCH COUNCIL OF CANADA LE CONSEIL NATIONAL DE RECHERCHES DU CANADA

Volume 53 Number 16 August 15, 1975 Volume 53 numCro 16 15 aoiit 1975

An Electron Spin Resonance Study of Some Reactions of Pentafluorosulfuranyl (SF,)

JOHN CHARLES T A I T ~ AND JAMES ANTHONY HOWARD Division of Chemistry, National Research Council of Canada, Ottawa, Canada KIA OR9

Received March 14, 1975

JOHN CHARLES TAIT and JAMES ANTHONY HOWARD. Can. J. Chem. 53,2361 (1975). A kinetic electron spin resonance study of the self-reaction of SF, and a spectroscopic and

kinetic e.s.r. study of the reaction of SF, with 1,l-di-t-butylethylene are reported. This radical undergoes self-reaction by a second-order process and the rate constants are given by the Arrhenius equation log 2kl(M-Is- ') = (10.3 + 0.5) - (1.7 + 0.5)/8 where8 = 2.303RTkcal mol-I. It adds to 1,l-di-t-butylethylene to give (~-Bu),~CH,SF, which decomposes by a first- order process with rate constants that obey the expression log k, (s-') = (13 f 0.4) - (10 f 0.2)/8. Both these rate constants are pertinent to kinetic studies of the photoinduced addition of SF5CI to olefins.

JOHN CHARLES TAIT et JAMES ANTHONY HOWARD. Can. J. Chem. 53,2361 (1975). On dBcrit I'etude cinetique par resonance paraBlectronique de I'auto-rkaction de SF, ainsi

que I'etude spectroscopique et cinttique par r.p.e. de la riaction de SF, avec le di--t-butyl-1,1 Bthylene. Ce radical reagit avec lui-m&me par un processus du second ordre. Les constantes de vitessesont donneespar1'Bquation d'Arrhtnius log 2kl (M-Is-') = (10.3 + 0.5) - (1.7 + 0.5)/8 oh 8 = 2.303RTkcal mol-I. Le radical s'additionne aussi sur le di-t-butyl-1,1 Bthylene con- duisant ainsi au ( ~ - B U ) ~ ~ C H ~ S F , . Ce produit se dicompose par un processus du premier order dont les constantes de vitesse obkissent a I'expression log k, (s-') = (13 f 0.4) - (10 + 0.2)/8. Ces deux constantes de vitesse sont pertinentes aux Btudes cinktiques de I'addition photo-induite du SF,CI sur les oltfines. [Traduit par le journal]

Introduction Pentafluorosulfuranyl (SF,) is one of the in-

termediates involved in the light induced and peroxide initiated addition of SF,Cl to olefins (1-4), in the radiolysis of SF, (5-7), and reaction of alkali metal atoms with SF, (8-10).

The electron spin resonance spectrum of SF, has recently been detected during liquid-phase photolysis of SF,Cl and by reaction of photo- chemically generated F. with SF, (11). This radical is a particularly interesting species from a theoretical standpoint (12) because the isotropic e.s.r. spectrum shows a hyperfine pattern due to

'NRCC No. 14686. 'NRCC Postdoctorate Fellow, 1974-1975.

the interaction of four equivalent fluorine atoms, the fifth fluorine showing no detectable isotropic hyperfine interaction.

Apart from some relative reactivity data ob- tained in the gas phase (2-4) little is known about the kinetics of reactions of SF, (and for that matter other sulfur centered radicals). We have, therefore, measured absolute rate constants for the self reaction of SF, in solution and rate con- stants for the p-scission of 1,l-di-t-butyl-2-penta- fluorosulfuranylethyl, (t-BU),CCH,SF,. The re- sults of this work are reported here.

Experimental Pentafluorosulfuranyl was prepared in situ in the cavity

of a Varian E-4 EPR spectrometer either by photolysis of

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2362 CAN. J . CHEM. VOL. 53, 1975

pentafluorosulfur chloride (Chemicals Procurement Laboratories Inc.) in a deoxygenated solvent (isopentane, dichlorodifluoromethane, and cyclopropane)

hv SF,Cl+ SF, + CI

or by photolysis of pentafluorosulfur peroxide (13) in the presence of phosphorus trifluoride in deoxygenated CF2Cl2.,

hv SF500SF5 + 2SF50.

SFSO. + PF, + [SF,OPF,] + SF, + OPF,

Absolute rate constants for self-reaction of SF, were determined by kinetic electron spin resonance spectro- scopy, k.e.s.r., (15) with the aid of a rotating sector and a Fabri-Tek 1072 Signal Averager. Steady state SF, con- centrations were determined by comparing the doubly in- tegrated first derivative spectrum with a similarly in- tegrated spectrum of a standard solution of DPPH. 1,l-Di-t-butyl-2-pentafluorosulfuranylethy radical was

prepared by photolysis of SF5C1 in isopentane or cyclo- propane containing 1,l-di-t-butylethylene. Decays of this species were followed by k.e.s.r. using the X-Y recorder provided with the spectrometer.

Results and Discussion SF5 + SF5

The concentration of SF, rapidly attained a steady state when the radical was generated by either of the methods described above. Steady- state radical concentrations were proportional to the square-root of the light intensity, indicating that the disappearance of SF, in these systems followed second-order kinetics, i.e.

The order of this reaction was confirmed by the linearity of plots of 1/[SF5] as a function of time after the initiating light was shuttered (Fig. 1).

Values of 2k1 were determined in three sol- vents, isopentane, cyclopropane, and dichlorodi- fluoromethane from 153 to 233 K. The values in cyclopropane and CF2C12 were identical, within experimental error, and were independent of the method of radical generation (see Table 1). The rate constants were found to fit the equation

where 0 = 2.303RT kcal mol-', as determined

3Photolysis of other peroxides in the presence of PF, gave alkoxytrifluorophosphoranyls, ROPF,, (14). How- ever, SF,O~F, is apparently too unstable to detect and SF, was the only species that could be detected.

FIG. 1. The reciprocal radical concentration as a function of time for the self-reaction of SF, in cyclopro- pane at 183 K.

from a least-squares fitting procedure on the Arrhenius plot.

These Arrhenius parameters are very similar to the values of A, = 10'' M-I s-' and E, = 0 predicted by Siddebottom et al. (2) and used by these workers to calculate Arrhenius parameters for addition of SF, to olefins.

Rate constants determined in isopentane were not reproducible and radical decays often de- viated from second-order. The reason for poor kinetic data in isopentane is not clear at present and rate constants obtained in this solvent were not included in the Arrhenius plot.

Extrapolation of the Arrhenius equation gives a value of 2k1 = 1 x lo9 M - l s - ' at 298 K somewhat less than the calculated diffusion- controlled limit for simple radicals in solution (16). A comparison of this value with absolute rate constants for self-reaction of other small radicals reveals that 2k1 is about 10 times smaller than the rate constant for CH, and about 10 times larger than the rate constant for CCl, (16). This difference between SF, and CCl, does, however, seem unreasonable. Relative rate con- stant measurements (17), which are inherently more accurate for comparing rate constants than individual rate constant measurements, indicate that the rate constant for self-reaction of CC1, is about 2 x lo9 M-'s-' which implies that

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TAIT AND HOWARD: E.S.R. STUDY OF REACTIONS OF SF, 2363

TABLE 1 . Rate constants for the self-reaction of SF,

Temperature [SF,], x lo7 2k, x Radical source Solvent c'c) (M) (M-I s-I)

CCl, and SF, have termination rate constants of a similar magnitude.

Values of 2k1 that can be calculated from the Arrhenius equation should be applicable to gas- phase systems involving SF, since there is no rea- son to believe that going from the liquid to the gas-phase would significantly influence the magnitude of this rate constant.

(t-BM) C C H ~ S F , Griller and Ingold (18, 19) have shown that

persistent 1,1 -di-t-butyl-2-substituted-ethyl radi- cals are readily obtained when a variety of free radicals are generated in the presence of 1,l -di-t- butylethylene, 1. Photolysis of SF,Cl in the pres- ence of 1 (5%) in isopentane from 133 to 173 K gave a 15-lined e.s.r. spectrum which could be interpreted as arising from the interaction of two equivalent hydrogen atoms with four equivalent fluorine atoms, consistent with the radical struc- ture (t-Bu),&H,sF,, 2. The agreement be- tween the observed and computer simulated spectra with a,(2) = 9.75 + 0.05 G, a,(4) = 14.12 & 0.05 G, g = 2.0032, and a linewidth of 3 G was excellent.

The absence of a hyperfine coupling from the fluorine trans- to the C-S bond is not surprising as it has been shown in several cases that the

coupling can be attributed to two effects. The steric effect of the SF, substituent will force the groups around the P-carbon into a locked con- formation in which the SF, group is eclipsed with the cr-carbon p-orbital containing the unpaired electron as is the case with other radicals of this type (21). This will tend to decrease the P-proton coupling from that which would be expected if there was free rotation about the C,-Cp bond. The further effect of an electron-withdrawing group on the P-carbon will withdraw electron density from these protons further decreasing the proton coupling. The large 33S hyperfine cou- pling observed is consistent with this explan- ation.

The steady-state concentration of 2 had an in- tensity exponent of 1.0 and radical decays were first order when the initiating light was shuttered, i.e.

(~-Bu),~CH,SF, must, therefore, decay by the unimolecular p-scission process

[21 2 + ( ~ - B U ) ~ C = C H ~ + SFs

rather than by the bimolecular disproportiona- tion

hyperfine coupling from this fluorine is very [31 2 , ( t - ~ U ) z ~ ~ ~ ~ 2 ~ ~ s + ( t - ~ u ) z ~ = ~ ~ ~ ~ , I small (<0.5 G) (1 1, 20). It was also possible to I observe the 3 3 ~ hyperfine coupling (a33s = 91 .7 Absolute values of k, were measured from 133

G) in natural abundance. Only the ++ transi- to 173 K and are summarized in Table 2. An I tions were observable, the + 3 transitions being Arrhenius plot of this data yielded the expression

hidden beneath the main spectrum. This log k, (s-l) = (13 + 0.4) - (10 + 0.218) I coupling, when compared with the 8n/3yey,$,,-

(0)' for 33S of 969 G, represents a 10% contribu- The magnitude of A , is consistent with a uni-

i tion from the sulfur 3s orbital to the unpaired molecular p-scission process and the low value , I electron orbital. of E, is in keeping with the reversible addition of

The magnitude of the P-proton hyperfine sulfur centered radicals to olefins (22,23). Sidde-

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2364 CAN. J. CHEM. VOL. 53, 1975

TABLE 2. Rate constants for the unimolecular decay of ( ~ - B U ) ~ ~ C H ~ S F ,

in isopentane

Temperature PC) kz (s-')

bottom et al. have in fact had to invoke the re- versibility of the addition of SF, to olefins to ac- count for the product distribution found for addition of SF,Cl to olefins (2-4).

SF,Cl was also photolyzed in the presence of ethylene and isobutylene. An e.s.r. signal could not be detected in isobutylene and although a signal was detected in ethylene it was too weak to permit analysis. The low intenisty of the e.s.r. signals in these olefins is probably because SF,Cl is rapidly consumed by a chain reaction. The over- all rate of reaction must be rapid at these low temperatures because even after a few minutes

2. H. W. SIDDEBOTTOM, J. M. TEDDER, and J. C. WALTON. Trans. Faraday Soc. 65,2103 (1969).

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5. K.-D. A s ~ u s , W. GRUNBEIN, and J. H. FENDLER. J. Am. Chem. Soc. 92,2625 (1970).

6. K.-D. A s ~ u s and J. H. FENDLER. J. Phys Chem. 72, 4285 (1968).

7. J. GAWLOWSKI and J. A. HERMAN. Can. J. Chem. 52, 3631 (1974).

8. H. C. COWEN, F. RIDING, and E . WARHURST. J. Chem. Soc. 4168 (1953).

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10. S. J . RILEY and D. R. HERSCHBACH. J. Chem. Phys. 58,27 (1973).

11. J . R. MORTON and K. PRESTON. Chem. Phys. Lett. 18, 98 (1973).

12. A. R. GREGORY. Chem. Phys. Lett.28,552(1974). 13. C. I. MERRILL and G. H. CADY. J. Am. Chem. Soc. 83,

298 (1961). 14. A. J. COLUSSI, J. R. MORTON, and K. F. PRESTON. J.

Phys. Chem. 79,651 (1975). 15. K. ADAMIC, D. F. BOWMAN, T. GILLAN, and K. U.

INGOLD. J. Am. Chem. Soc. 93,903 (1971). 16. K. U. INGOLD. Free radicals, Vol. 1. Edited by J. K.

Kochi. Wiley, New York. 1973. Chapt. 2. 17. D. G ~ U L L E R ~ ~ ~ K. U. INGOLD. Int. J. Chem. Kinet. 6,

453 (1974). 18. D. G ~ U L L E R ~ ~ ~ K. U. INGOLD. J. Am. Chem. Soc. 95,

pho~olysis, SF,Cl and the olefin were converted 6459 (1973).

almost quantitatively to SF,CR,CH,C~ and 19. G. D. MENDENHALL, D. GRILLER, D. LINDSAY, T. T. TIDWELL, and K. U. INGOLD. J. Am. Chem. Soc. 96,

SF,CH,C(CR,),CI, as shown by the correspon- 2441 (1974). dence of the 1 9 ~ and n.m.r. spectra of the 20. J . R. MORTON and K. F. PRESTON. J. Chem. Phvs. 58. - . products with those analyzed previously by 2657 (1973).

Boden et (24). In the case of di-t-butyIethylene, 21. D. GruLLERand K. U. INGOLD. J. Am. Chem. Sot. 96, 6715 (1974).

no 19F resonance was observable, indicating that 22, C. WALLING, Free radicals in solution, Wiley, New 2 is too sterically hindered to abstract a chlorine York, New York. 1957. atom from SF,Cl. 23. W. A. PRYOR. Free radicals. McGraw-Hill, New

York, New York. 1966. 1. R. E. BANKS, R. N. HASZELDINE, and W. D. 24. N. BODEN, J. W. EMSLEY, J. FEENEY, and L. H.

MORTON. J. Chem. Soc. C, 1947(1969). SUTCLIFFE. Trans. Faraday Soc. 59,620 (1963).

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